# `Placement Driver (PD)`

__*zz-Issue-Watch*__

+ Logical single point of the entire system
+ `Leader` provides serivces, all `followers` are standby
+ Consistent requirements
  + Allocate strictly increasing timestamps
  + There can be only one `leader` at a time
+ Rely on embedded `etcd` to implement `leader` election
  + **`Raft`**
  + **`lease`**
  + **`watch`**
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A `TSO` is based on `PD` leader's local time and a strictly increasing logical time, a `Unix` timestamp has 10-digits, but a `TSO` has 18-digits.